1. Field of the Invention
The present invention relates to a wireless communication apparatus having a receiver which demodulates a phase-modulated received signal and a transmitter which transmits a phase modulation signal to the receiver.
2. Related Art
In a mobile wireless communication field such as a portable phone, there have been proposed many kinds of radio frequency transmission/reception schemes. One of their schemes is a superheterodyne scheme which has been practically used from a long time ago. In the superheterodyne scheme, the received radio frequency signal is converted into an intermediate frequency signal, and then a certain channel frequency signal is selected to perform quadrature demodulation. The quadrature-demodulated signal passes through a low pass filter and an A/D converter in sequence to perform the respective processings, and then is demodulated.
Recently, a mainstream scheme in a GSM cellular phone system and the like is a direct conversion scheme (see Japanese Patent Laid-Open. Pub. No. 136045/1998). In this scheme, the received signal passes through a radio frequency bandwidth filter, and then is separated into two paths. Subsequently, the separated signals are mixed with local oscillation signals having phases orthogonal to each other by a quadrature demodulator to generate a baseband signal. The baseband signal passes through the low pass filter and the A/D converter before being demodulated. The transmitted signal is dealt with processings contrary to the received signal.
Both of the superheterodyne scheme and the direct conversion scheme have to perform the processing of converting the radio frequency signal generated by a local oscillator into the intermediate frequency, the quadrature demodulation processing, the demodulation processing after A/D conversion and the like. Therefore, configuration of the system is complicated.
In order to generate the local oscillation signal, some components such as a TCXO (Temperature Compensated Crystal Oscillator) have to be externally attached. Therefore, downsizing is difficult and a cost of components becomes expensive.
As a future wireless system, there has been proposed a system in which A/D conversion is performed by an oversampling scheme directly using a carrier wave frequency to select and demodulate the channel signal by digital processing.
However, in an extremely high carrier wave frequency such as 800 MHz to 2 GHz used by the cellular phone, an extremely high-speed A/D converter has to be provided to perform A/D conversion by the oversampling. Therefore, power consumption increases more than several dozen watts and it is difficult to practically develop such an A/D converter in the near future.